Diddy Kong is like Luigi Jr. with with bananas; he has good aerials for racking damage, throws to put you in a bad position so that he may take great advantage of it, and a variety of methods of recovery so that he can mix it up. Without bananas, Diddy is not a very strong character. That's what this MU is about; we need to treat this MU like the Ice Climbers and cut Diddy Kong off from his best quality, which would be his bananas. Luckily, Yoshi has options to do such a thing, but also like the ICs, we must do it very carefully.
In the beginning of the match, Diddy will start out 75-80% of the MU doing a full-hop back and plucking out a banana; the remaining % consists of him just pressing down-b immediately. Yoshi wants to take advantage of this time to just throw an egg straight at Diddy, or rather, try to throw it so that it will hit him during either option. This is just so that we can startle him and allow a free advancement towards getting in on him.
Although I already said Diddy Kong is like Luigi Jr. with bananas, Yoshi is like Luigi as well; a great jab, great aerials, throws that can put opponents in the right position in order to cut off their options and take advantage of the position, and lastly, has a great tactic against Diddy Kong. In the Luigi v. Diddy Kong MU, Luigi can use a tactic to deal with Diddy Kong in one of the best ways possible:
"Get in between Diddy and his bananas and say, 'Do something!'"
That's what we're going to do, and like I said what Yoshi has what Luigi doesn't in my summary of the Marth MU, we have the aerial mobility and overall maneuverability to get in and sever Diddy's metaphorical arteries, the key to how Diddy works.
I have noticed that people are talking about how the CG isn't as useful in this MU, due to the threat of tripping. That is not the point of getting a grab on Diddy Kong; if a banana is in front of you as you grab Diddy, go for a back-throw because that is exactly the position that you want to attain in this MU. Sending him away from his bananas as you advance on taking advantage of them forces him to either fight you head on without bananas, which is something that I see in our advantage; or if you choose to throw the bananas away offstage, forces him to repluck the bananas, leaving him VERY open.
When you glance towards mid-high level Diddys, they are often too focused on replacing their peels that they will leave themselves open for free damage when they are caught in the ending lag. Now as you scale towards the top-level players, they will not pluck until you are on the complete other side of the stage, or are even offstage. This is a time when Diddy has gained the advantage of the match, so you do not want to be caught in mid range when his ending lag is over.
I'll return to comparing Diddy Kong to Ice Climbers, or even Marth, in that Diddy's strongest asset will force you to reset if you can manage to escape, like breaking out of ICs grabs or getting hit by Marth in general. In order to reset, I recommend just running away outside of Diddy's item toss range. If you try to fight back after every throw, aerial, etc., following from a trip, YOU WILL GET F*CKED UPPPPPPP.